What crowds a letter in the periphery?

Abstract

In the periphery, nearby letters (flankers) can impair the identification of a target letter, but white-noise patches or gratings do not (Palomares et al., ARVO99), suggesting that letter crowding is unlike contrast masking (Chung, Levi, Legge, 2001). The aim of this study was to systematically test different types of flankers that may induce letter crowding in the periphery. Subjects identified a target letter presented at 5 deg inferior to fixation. Letters were in Times New Roman, of size 0.4 log units above the subject's acuity. Six conditions were ested: target letter alone, flanked by same-polarity letter flankers, opposite-polarity letter flankers, letter- noise patches, or white-noise patches at 1 x-height spacing, or flanked by same-polarity letter flankers at 2 x-height spacing. The letternoise patch was created by scrambling the phase spectrum of a letter but retaining its power spectral density (PSD). Moreover, both types of noise had the same contrast energy and bounding box as the letter they replaced. Flankers were at 20% contrast. Threshold contrast of the target letter was determined at 79% correct. Average threshold contrast (across 3 Ss) for the target-alone condition was 12.8% (+/-0.04 log units). No significant threshold elevation was observed for white-noise patches or letter flankers at 2x spacing. All other conditions led to significant crowding (avg. threshold elevation in log units - letter: 0.35, letter noise: 0.26, reverse contrast: 0.21). Threshold elevation between letter and letter noise flankers was not significant for 2 out of the 3 Ss. Letter-noise patches caused significant crowding, while white-noise patches did not. Spatial frequency distribution thus seems to play a major role in letter crowding. Phase, however, which defines the visual form of the flankers, had only a limited role. We speculate that a main component of letter crowding may be noise masking, with noise being induced by and having a similar PSD to the flankers.